The present invention relates a conductive adhesive which develops conductivity through the medium of an electrolyte contained therein and, more particularly, to a conductive adhesive suited for use as a biomedical electrode.
A biomedical electrode is usually composed of a conductive layer to be made in contact with the skin of mammals (including human) and an electrode connected electrically with the conductive layer, and is used in various applications depending on its construction. For example, a biomedical electrode can be used for electrical diagnosis, treatment or surgery of mammals. As a device for measuring a cardiac action potential, for example, the biomedical electrode is applied to the surface of the living body. In such a case, the biomedical electrode detects a change in potential on the surface of the living body in an electrocardiograph for measuring and recording a cardiac action potential. A change in potential on the surface of the living body, detected by such a biomedical electrode, is inputted into an electrocardiograph equipped with the biomedical electrode, recorded as an information of an electrocardiogram, and then displayed or printed. A terminal of the biomedical electrode is connected to the electrocardiograph through a connection cable or lead wire.
The above-described biomedical electrodes can comprise, for example, an Ag/AgCl conductor eyelet, a stud of carbon (or metal) as an electrode terminal, and a conductive non-adhesive gel layer as a conductive layer to be placed in contact with the skin surface. Such a conductive non-adhesive gel is disclosed, for example, in Japanese National Publication (Kohyo) No. 56501108; or corresponding U.S. Pat. No. 4,406,827 and Japanese Utility Model Registration No. 2570183. It is necessary that such a biomedical electrode further comprises an adhesive tape (backing tape) for fixing it to the surface of the living body. Biomedical electrodes that include a conductive adhesive layer as a conductive layer, however, do not require the adhesive tape.
A conductive adhesive, which can be used as the above-described conductive adhesive layer, is disclosed in Japanese Examined Patent Publication (Kokoku) No. 8-19394 and U.S. Pat. No. 4,524,087. A biomedical electrode utilizing a conductive adhesive is disclosed in U.S. Pat. No. 5,078,139. Such biomedical electrodes can be fixed to the surface of the living body without containing a backing adhesive tape for fixing, and can comprise:
(a) a conductive adhesive layer containing an aqueous electrolyte solution and an adhesive polymer,
(b) a liner for coating one surface (adhesive surface) of the conductive adhesive layer,
(c) a backing for coating the other surface of the conductive adhesive layer, and
(d) an electrode terminal connected with the conductive adhesive layer, which has an exposed portion coated neither with the liner nor backing.
When this biomedical electrode is applied to the skin, the electrode can be readily fixed only by peeling off the liner to expose one surface of the conductive adhesive layer, and bringing the adhesive surface into contact with the surface of the living body thereby to slightly contact-bond them. A conventional conductive adhesive layer has sufficient initial adhesive strength, but as a result of absorption of sweat from the skin into the adhesive layer the adhesive strength is likely lowered over time.
U.S. Pat. Nos. 5,779,632 and 5,670,557 disclose a so-called xe2x80x9cbicontinuous conductive adhesivexe2x80x9d having a continuous structure comprising a hydrophilic conductive phase containing an aqueous electrolyte solution and a hydrophobic adhesive phase. In such a bicontinuous conductive adhesive, the hydrophilic phase containing an electrolyte is a continuous layer and ionic conductivity can be exhibited. Since the hydrophobic adhesive layer has a bonding function and sweat from the skin is absorbed by the hydrophilic phase, lowering of the adhesion strength can be improved. In the bicontinuous conductive adhesive (or biomedical electrode using the same), however, it was particularly difficult to retain water content during storage under low humidity conditions. Accordingly, the biomedical electrode must be stored in a sealed pouch to prevent drying (vaporization) of the phase including the aqueous electrolyte. It is also normally necessary to use the biomedical electrode within 10 to 30 days after opening the sealed pouch since, it becomes difficult to obtain sufficient electrical characteristics (i.e., sufficiently low impedance) when the conductive adhesive dries.
U.S. Pat. No. 5,338,490 discloses an adhesive comprising (A) a first phase containing a hydrophilic polymer and an aqueous solution of electrolyte, and (B) a second phase containing a hydrophobic adhesive polymer, wherein the first phase is a continuous phase and the second phase is a domain phase contained in the state of being dispersed in the first phase.
U.S. Pat. No. 5,270,358 discloses a so-called xe2x80x9cbidispersedxe2x80x9d adhesive composition comprising a continuous phase of a hydrophobic pressure-sensitive adhesive and a hydrophilic dispersed phase of a hydrogel having pressure-sensitive adhesion characteristics. However, this adhesive composition has no ionic conductivity because the hydrophilic dispersed phase (each particle of hydrogen) is not a continuous phase and, therefore, the adhesive composition can not be used as the conductive adhesive.
It is desirable to include a humectant in the conductive adhesive of a biomedical electode to impede drying of the conductive adhesive. However, when high-performance humectants such as amino acids are added to the bicontinuous conductive adhesive to obtain a high moisturizing effect, an expected two-phase structure can not be obtained. That is, when a comparatively large amount of the humectant exists, a continuous structure of a hydrophobic adhesive phase is likely broken into an emulsion as a raw material of the conductive adhesive, thereby making it difficult to maintain the structure in an effective state (e.g. a state where sufficient adhesion property can be exhibited).
In one aspect the present invention provides a conductive adhesive comprising:
(A) a first phase containing a hydrophilic polymer, an aqueous electrolyte solution and a humectant, and
(B) a second phase containing a hydrophobic adhesive polymer, characterized in that:
the first phase is a continuous phase and the second phase is a domain phase that is dispersed in the first phase, and the domain phase has an average diameter within a range from 0.02 xcexcm to 1 mm.
In another aspect, the present invention provides a biomedical electrode comprising an adhesive layer containing the conductive adhesive of the present invention, and an electrode terminal connected with the adhesive layer.